Power converters are used to convert electric power from one form to another, for example, to convert direct current (DC) power to alternating current (AC) power. One important application for power converters is in transferring power from energy sources such as solar panels, batteries, fuel cells, etc., to electric power distribution systems such as local and regional power grids. Most power grids operate on AC current at a line (or mains) frequency of 50 or 60 cycles per second (Hertz or Hz). Power in an AC grid flows in a pulsating manner with power peaks occurring at twice the line frequency, i.e., 100 Hz or 120 Hz. In contrast, many energy sources supply DC power in a steady manner. Therefore, a power conversion system for transferring power from a DC source to an AC grid typically includes some form of energy storage to balance the steady input power with the pulsating output power.
This can be better understood with reference to FIG. 1 which illustrates the mismatch between a DC power source and a 60 Hz AC load. The amount of power available from the DC source is shown as a constant value at the center line of the sine wave. In contrast, the amount of power that must be transferred to the AC load is shown as a sine wave that fluctuates from the zero power level at the minimum of the sine wave to a maximum value and back down to minimum once every half line cycle. For a system with a grid frequency fgrid, a half line cycle is given by 1/(2*fgrid), which is 10 milliseconds (ms) 50 Hz systems, and 8.33 ms for 60 Hz systems. During time T1, the power available from the DC source exceeds the instantaneous power required by the AC load. During time T2, however, the maximum power available from the DC source is less than that required by the load. Therefore, to effectively transfer power from the source to the load, the power conversion system must store the excess energy from the power source during time T1 (shown as the shaded area S), and discharge the stored energy to the load during time T2 (shown as the shaded area D).
FIG. 2 illustrates a conventional system for converting DC power from a photovoltaic (PV) panel to AC power. The PV panel 10 generates a DC output current IPV at a typical voltage VPV of about 35 volts, but panels having other output voltages may be used. A DC/DC converter 12 boosts VPV to a link voltage VDC of a few hundred volts. A DC/AC inverter 14 converts the DC link voltage to an AC output voltage VGRID. In this example, the output is assumed to be 120VAC at 60 Hz to facilitate connection to a local power grid, but other voltages and frequencies may be used.
The system of FIG. 2 also includes a DC link capacitor CDC and a decoupling capacitor C1. Either or both of these capacitors may perform an energy storage function to balance the nominally steady power flow from the PV panel with the fluctuating power requirements of the grid. Power ripple within the system originate at the DC/AC inverter 14, which must necessarily transfer power to the grid in the form of 120 Hz ripple. In the absence of a substantial energy storage device, this current ripple would be transferred all the way back to the PV panel where they would show up as fluctuations (or “ripple”) in the panel voltage VPV and/or current IPV. Therefore, the DC link capacitor CDC, or less often, the decoupling capacitor C1, is used to store enough energy on a cycle-by-cycle basis to reduce the ripple at the PV panel to an acceptable level.
Ripple on a link capacitor also affects the downstream operation of the system. FIG. 3 illustrates the instantaneous demand for voltage from an H-bridge type DC/AC inverter in comparison to the voltage available from a DC link capacitor that is maintained at a fixed voltage. As long as the DC link voltage is maintained above the peak voltage demand from the inverter (plus an extra amount for headroom), the inverter can produce the AC output with little or no harmonic distortion (HD) in the output voltage and current waveforms. Therefore, reducing ripple on the link capacitor is also beneficial from the perspective of the downstream operation of the system. The ripple can be reduced by using a larger capacitor, but increasing the size of a capacitor dramatically increases its cost.
Moreover, in conventional systems, energy storage capacitors tend to be problematic components for several reasons. First, a capacitor that is large enough to provide adequate energy storage must generally be of the electrolytic type, since other large capacitors are usually prohibitively expensive. Electrolytic capacitors, however, have limited life spans and tend to have a high failure rate. As a further complication, the capacitance of an electrolytic capacitor steadily decreases over its lifetime as the electrolyte dissipates and/or deteriorates, thereby reducing its effectiveness and changing the dynamics of the entire system. Further, electrolytic capacitors tend to be bulky, heavy and fragile, and have a large equivalent series resistance (ESR). Thus, the capacitor in a conventional power conversion system is often the weakest link.
In a relatively recent development, gains in reliability and other characteristics have been achieved by taking the opposite approach to sizing the link capacitor. See, e.g., U.S. Patent Application Publication Nos. 2010/0157638 and 2010/0157632 which are incorporated by reference. In such systems, rather than maximizing the size of the link capacitor to minimize the ripple, a smaller link capacitor is used, and the voltage on this capacitor is purposely allowed to vary over a relatively wide range as shown in FIG. 4. Relaxing the ripple voltage range in this manner may enable the size of the link capacitor to be reduced because the amount of energy stored in a capacitor is directly related to the voltage swing across the capacitor. This may also make it economically viable to use more reliable types of capacitors such as those having metal film or other non-electrolytic construction.
As long as the minimums in the voltage available from the link capacitor do not drop below the voltage demanded from the inverter, distortion in the output voltage and/or current from the inverter can be held to an acceptable level using advanced algorithms such as those disclosed in the U.S. patent application Publications referenced above.
FIG. 5 illustrates a prior art control loop for regulating the voltage on a link capacitor in a system having relaxed ripple voltage requirements. A control signal CTRL controls a power stage 16 which in turn causes a certain amount of ripple on a DC link capacitor 18. From the perspective of power flow, the power stage may be arranged before or after the link capacitor. From the perspective of the control loop, however, the power stage has a causal effect on the link capacitor voltage VDC, which is filtered by a low-pass filter 20 to generate an average value VAVE that is compared to a reference signal REF at a nulling circuit 22 to generate the control signal CTRL.
The low-pass filter 20 has a cut-off frequency that is substantially lower than the ripple frequency of the DC link capacitor. For example, in a 60 Hz power system the capacitor experiences a 120 Hz ripple, so the cut-off frequency of the low-pass filter 20 may be set to about 240 Hz to filter out the harmonic distortion and provide a sine wave of 120 Hz.
The reference signal REF is applied as a fixed or slowly varying signal with a time constant that is longer than the time constant of the low pass filter. The control loop attempts to balance the input and output power by controlling the output power injected into the load to match the available input power. The power withdrawn from the link capacitor is controlled so as to maintain an average target voltage across the capacitor. This average is maintained by reducing or increasing the current out of the capacitor into the DC/AC inverter. The minimum voltage across the capacitor must always be greater than the grid voltage in order for the inverter circuit to work without distorting the power injection onto the grid. In addition, the maximum voltage across the capacitor must be less than the rated capacitor voltage, with some additional safety margin to increase reliability of the system. Thus, the control loop causes the link capacitor voltage to slide up and down on a pedestal, which is the average value VAVE of the link capacitor voltage, to satisfy the requirements for power balance, safety margin, etc.
A problem with the conventional approach is that the low-pass filter causes long delays in the feedback loop, so the system is slow to adapt to unpredictable changes in input power (e.g., solar) or output power (e.g., load connection/disconnection). If the capacitor voltage exceeds one of the safety margins, the input power may need to be switched off to avoid a capacitor overvoltage situation, or the load current may need to be reduced to avoid an under-voltage situation.
If power into the system increases suddenly, or the output load decreases (i.e. the inverter output is disconnected), the system cannot respond fast enough, so a fast capacitor over-voltage condition occurs. One conventional response to this potentially destructive situation is to implement a fast over-voltage protection system to protect the capacitor. This response, however, increases the cost of the system and can significantly increase power dissipated within the inverter if it is used frequently.
Another conventional response to a capacitor over-voltage condition is to completely switch off input power. Although this type of complete shut down typically takes longer to achieve, it can effectively neutralize an on-going uncorrectable power imbalance. However, switching off the power source may incur a large delay before power can be reapplied because the control loop cannot control a sudden application of input power due to its slow speed. Instead, a gradual power-up ramp is typically used. Moreover, in grid-tie inverter situations, once the inverter is disconnected from the grid, reconnection may require a mandatory waiting period of several minutes. This leads to reduced energy harvest from the power source such as a photovoltaic array.
Likewise, if the power into the system drops suddenly, or an output load increases, the slow response of the control loop causes a fast capacitor under-voltage condition to occur, thereby causing distortion of the power injected onto the grid. While such a situation is typically not critical, it may cause the inverter power injection distortion to be non-compliant with regulations if it happens too frequently.
In either case, greater DC link capacitor voltage margins may be used to compensate for the slow system response. However, this requires a DC link capacitor with a higher voltage rating which is more expensive. Higher DC link voltages may also increase the costs of other system components.